This is the A-List:
Aaaand we're done.
Really, that's it. Marvel/Disney is working hard to change that, but Iron Man and Thor are not (yet) A-Listers. Even Wolverine, who's a huge fan favorite, is not an A-List superhero that everybody knows. I bet most Americans can't name three X-Men, and that includes Professor X. Until Guardians of the Galaxy came out, even I didn't know the names of any of its protagonists (though I did recognize Thanos).
There are a few others that might be borderline:
These are the sorts of characters that people outside the fandom know exist. But they don't care or know much about them. I didn't know Captain America's origin story until I saw the movie. Three of them are "the girl version of an A-lister", which as a reason for mattering is meh. Catwoman is known as a supervillain or at best an antihero.
Other candidates? Fantastic Four? They just don't have the kind of presence in the popular imagination that Superman or even Spider-Man does.
I kinda want to see market research on this, but asking my friends doesn't work as well on this one, because even most of you who never read superhero comics are still peripherally involved with the fandom. :) Still, I'm curious which heroes you think are on the A-List, or if anyone feels like the recent success of various Avengers films has moved any of those characters onto it.